The present application hereby claims priority under 35 U.S.C. xc2xa7119 on German patent publication number 10142605.4 filed Aug. 31, 2001, the entire contents of which are hereby incorporated herein by reference.
Imaging data from an examined test object can be obtained with modern medical diagnostic techniques, for example the computed tomography (CT). As a rule, the test object to be examined is a patient.
The imaging data are generally obtained in digital form and are thus accessible for the digital data processing. Typical operations of the digital data processing are, for example, the intensification, super-imposition or filtering.
In addition to the desired imaging information from the examined test object, the imaging data obtained by use of imaging systems for medical diagnostics normally contain information that can be traced back to interfering influences, such as parasitic induction, during the testing procedure.
In general, the problems are divided into two different categories that reduce the quality of the obtained imaging data: image background noise and artifacts.
In the following, these two problems are to be explained further with the example of the computed tomography.
The image background noise in turn can be divided into several causes.
The main share of the image background noise results from the quantum noise, caused by the fact that each emission of rays is composed of a finite quantum number, so that the quantum number that is measured always fluctuates with normal distribution around an average value.
Further causes for the image background noise are the monochromatic quantum, which are for the most part not exact, of X-ray tubes that can realistically be produced, as well as the scattering of rays based on the interaction between the X-ray radiation used and the electron sheath of atoms during the transmission through the test object.
The artifacts can also be divided further.
Aliasing, partial-volume artifacts, age-hardened artifacts, as well as movement artifacts are typical types of artifacts, the appearance of which in particular depends on the geometry or a movement by the test object.
Effects corresponding to the image background noise, described in the above with the computed tomography, and the artifacts can also be found in other imaging system for medical diagnostics.
Ring artifacts are a special form of the artifacts, caused primarily by the imaging systems themselves that are used for the medical diagnostics.
Imaging systems for medical diagnostics which use (for example in modern computed tomography machines) several detectors (in particular also several detector layers) provide the option of insufficiently calibrating the individual detectors. This is understood to mean that different detectors will measure the same amount of weakening of the rays penetrating the test object differently.
With an insufficient calibration of the individual detectors, for example in a computed tomography machine, the obtained imaging data show concentric rings around the rotational center of the computed tomography machine because of the rotation of the radiation source and the detectors around the test object during the measuring operation. These rings do not have an actual relationship to the test object that is examined. Interferences of this type in the imaging data are called ring artifacts.
Since the manufacturer of an imaging system for medical diagnostics normally calibrates the detectors, the frequency and clarity of ring artifacts is an indicator for the presentation quality of such a system. A low number and low clarity of the ring artifacts indicate great care taken by the manufacturer during the testing and calibration of the imaging system and thus a high quality.
Imaging systems of this type for medical diagnostics are therefore subjected to a final testing by the manufacturer before the system is delivered. Within the framework of this final testing, at least one test image is created for each detector layer with the aid of a test body having a known absorption coefficient (such a test body is also called a xe2x80x9cphantomxe2x80x9d). As with many other calibration methods, the number of tests performed (test images created) also increases the accuracy. The obtained test images are then checked visually by specially trained personnel for the presence of ring artifacts and the individual detectors are calibrate accordingly.
The disadvantage of this known method, however, is its high cost. Modern CT systems, for example, may require the visual checking of several hundred test images for each system.
German Unexamined Published Patent Application No. 31 07 170 A1 discloses a device for reducing ring-shaped artifacts arranged concentrically around the center of the area to be examined by a computed tomography machine. These artifacts are superimposed on the reconstruction image of the computed tomography machine.
A smoothing device, such as a filter, is proposed for this which smoothes the image values of the reconstruction image in an azimuth direction, relative to the center of the examined area (for example through low-pass filtering). As a result, the ring artifacts are emphasized more in the reconstruction image.
The ring artifacts are subsequently detected in the reconstruction image in a ring-detection stage through filtering along straight lines through the center of the examined area. A median filter, with a filter length that is double the width of the artifacts plus an image point (pixel) is suggested for the filter. By subtracting the image values filtered with the median filter from the smoothed image values, correction image values are computed for all image values.
The computed corrected image values are subsequently compared to a minimum threshold value and a maximum threshold value. On the one hand, this serves to suppress the ring artifacts that cannot be detected visually. On the other hand, it ensures that elements not corresponding to ring artifacts, but some other type of object structure (e.g. a bone structure), are not corrected by mistake.
The disadvantage of the known method and device is that in order to compare the detected ring artifacts to a previously selected criterion, a meridian filtering must first be carried out. Meridian filtering operations of this type are involved computations since they require a sorting function and thus iterative steps. Furthermore, it is necessary to have prior knowledge of the width of the artifacts to determine the filter length.
Furthermore, the correction value used for evaluating the visual detectability of the detected ring artifacts according to German Patent 31 07 170 A1 is a poor indicator for the visual detectability of the detected ring artifacts. This is because it is only an indicator for the expansion of the image values within the filter length and is not a direct indicator of their clarity.
In addition, the meridian filter according to this prior art is used for detecting the ring artifacts as well as for evaluating them. This results in a high dependence on the selected meridian filter as well as the selected filter length. Thus, the result is only meaningful to a limited degree as it relates to the visual detection of the ring artifacts.
It is the object of an embodiment of the present invention to provide a method and a device for checking the presentation quality of an imaging system for medical diagnostics with respect to ring artifacts, for which the visual detectability of ring artifacts can be evaluated reliably and with little expenditure.
According to an embodiment of the present invention, a method for checking the presentation quality of imaging systems for medical diagnostics with respect to ring artifacts is divided into the following steps:
Creating at least one test image; Defining concentric circles in the at least one test image, wherein the definition of the concentric circles in the at least one test image occurs around a rotational center which is fixed in the test image by the imaging system for medical diagnostics that is used; Averaging the test image points, arranged on the respective circumferential lines of the circles in circumferential direction of the respective circle for creating a ring profile; Detection of ring artifacts in the created ring profile; Comparing the detected ring artifacts to a pre-selected criterion for evaluating a visual detectability of the detected ring artifacts, wherein the detected ring artifacts are compared to the pre-selected criterion by computing the visual signal noise for each ring artifact and comparing the computed signal noise to the pre-selected criterion.
According to an embodiment of the invention, concentric circles must first be defined in the at least one test image before the image points arranged on the circumferential lines of the concentric circles can be averaged. Thus, according to an embodiment of the invention, it is possible to process the imaging data upon which a test image is based in such a way that existing ring artifacts are clearly emphasized based on a reduction of the other image information.
The ring artifacts emphasized in this way are particularly easy to detect and can be evaluated with respect to their visual detectability with the aid of the pre-selected criterion.
For this, an embodiment of the invention suggests comparing the detected ring artifacts with the pre-selected criterion through computing the visual signal noise for each ring artifact and comparing the computed signal noise to the pre-selected criterion.
The visual signal noise of the respective ring artifacts can be computed simply and without great computation expenditure since no iterative steps are required for computing the visual signal noise.
Furthermore, the visual signal noise of the respective ring artifacts is an excellent indicator for the visual detectability of the respective ring artifact since it directly indicates the intensity of each ring artifact.
The criterion to which the computed visual signal noise for evaluating the visual detectability of the ring artifacts is compared represents a measure for evaluating the visual detectability of the detected ring artifacts.
The advantage of using such a criterion is that is allows, for example, an adaptation to the different imaging systems for medical diagnostics or to different test objects used to create the test image.
The method according to an embodiment of the invention thus makes it possible in a particularly simple and reliable manner to evaluate the visual detectability of ring artifacts detected in a test image.
Since the frequency and clarity of ring artifacts in a test image also indicates the presentation quality of an imaging system for medical diagnostics, the quality of such a system can easily be checked with an evaluation of the visual detectability of detected ring artifacts.
For an automatic detection and/or evaluation of ring artifacts in the test image, it is advantageous if a pixel noise that depends on the respective radius of the respective circle is detected at the same time as the ring profile for each circumferential line of the circles is created and if the visual signal noise for each ring artifact is computed with the following formula:   SNR  =      α    ·                            ∑                      r            =                          r              ⁢                              xe2x80x83                            ⁢              min                                            r            ⁢                          xe2x80x83                        ⁢            max                          ⁢                  xe2x80x83                ⁢                              R            r                    ⁢                      B            r            2                                                            ∑                          r              =                              r                ⁢                                  xe2x80x83                                ⁢                min                                                    r              ⁢                              xe2x80x83                            ⁢              max                                ⁢                      xe2x80x83                    ⁢                                    R              r                        ⁢                          B              r              2                        ⁢                          s              r              2                                          
wherein
SNR is the visual signal noise of the respective ring artifact to be computed
xcex1 is a constant
Br is the ring profile of the test image that is averaged along the respective circumferential lines of the circles,
Rr is the radius for the circumferential line of the respective circle, along which the test image is averaged,
Sr is the pixel noise depending on the respective radius Rr,
r is an index that describes with an appropriate grid (scanning grid) the radius Rr and
rmin, rmax are the start and end radii of the various detected and potentially visible ring artifacts.
The pixel noise depending on the respective radius of the respective circle can thus preferably be determined during the averaging of the image points arranged on the circumferential line of the respective circle.
The constant xcex1 can be equal to 1, but preferably assumes the value xcex1={square root over (2xcfx80)} for standardizing the visual signal noise.
It is preferable if the ring profile is subjected to a low-pass filter for noise suppression prior to the detection of the ring artifacts and/or a high-pass filter for suppressing long-wave interferences since ring artifacts in a ring profile processed in this way are even easier to detect.
According to one preferred embodiment, the ring artifacts in the ring profile are detected through a detection of the mathematical sign change in the ring profile.
To check the quality of the image representation in imaging systems for medical diagnostics with the aid of a uniform scale, it is furthermore advantageous if the threshold value for the visual detection of the ring artifacts is determined in another method step and the threshold value is compared to the evaluated visual detectability of the detected ring artifacts.
Determining a threshold value for the visual detectability of the ring artifacts not only permits the evaluation of the visual detectability of the detected ring artifacts with the above-described criterion, but also allows checking the evaluated visual detectability of the detected ring artifacts with respect to the highest permissible value for the visual detectability of the detected and evaluated ring artifacts. This highest permissible value, for example, can depend on the imaging system for medical diagnostics to be checked, or can be predetermined client-specific in the form of specified values.
The threshold value for the signal noise of the respective ring artifact is preferably fixed since the signal noise for all ring artifacts can be computed easily with the aid of the above provided formula.
For one preferred embodiment, the presentation quality of the imaging system for medical diagnostics to be checked is judged to be insufficient if the visual detectability or the signal noise of the respectively detected ring artifacts is higher than the threshold value.
When checking imaging systems for medical diagnostics, it is an advantage based on better standardization if the at least one test image can be created with the aid of a homogenous phantom, meaning with the aid of a test body with known absorption coefficient. This homogeneous phantom can be a water phantom for example.
Another advantage when using a phantom to create the at least one test image is that interfering influences based on the nature of the test object can be avoided, or that these influences occur in the same way for all imaging systems for medical diagnostics that must be checked.
The method according to an embodiment of the invention is particularly good for checking the presentation quality of images created with computed tomography machines since these have a high number of detector layers. As a result, checking the image presentation quality of computed tomography machines by use of a conventional visual evaluation of test images with trained personnel is particularly expensive and involved owing to the high numbers of test images.
According to an embodiment of the invention, a device for realizing the method according to an embodiment of the invention is also provided and comprises the following components:
A defining device for defining concentric circles in the at least one test image created by the imaging system for medical diagnostics to be checked, wherein the definition of the concentric circles in the at least one test image occurs around a rotational center which is fixed in the test image by the imaging system for medical diagnostics used;
An averaging device that is designed to average the image points of the test image, arranged on the respective circumferential lines of the circles, for creating a ring profile in circumferential direction;
A detector that is designed to detect ring artifacts in the created ring profile; and
A comparator that is designed to compare the detected ring artifacts for evaluating a visual detectability of the detected ring artifacts to a pre-selected criterion, wherein the comparator is further modified to compute the visual signal noise for each ring artifact and to compare the computed signal noise to the pre-selected criterion.
An embodiment of the present invention also relates to a computer program product that is suitable for realizing the method if loaded into a memory of a processor.